1. Field of the Invention
The present invention relates to a rotary servovalve adapted for use in a power-assisted steering system, and more particularly to a rotary servovalve of the constant flow type which is arranged to constantly permit the flow of fluid under pressure passing therethrough from its inlet port to its discharge port when a steering wheel is held in a neutral position.
2. Description of the Prior Art
In practical use of such a rotary servovalve as described above in a power-assisted steering system, hydraulic pressure supplied from a fluid pump is maintained at a low level defined by passage resistance when the steering wheel is held in a neutral position and increases in accordance with an increase of load acting on the fluid pump when the steering wheel is turned. This is useful to reduce power consumption caused by operation of the fluid pump and to provide hydraulic assist power for smooth operation of the steering wheel. In the rotary servovalve, however, distribution ports in connection to opposite fluid chambers of a hydraulic power cylinder are communicated with inlet and discharge ports when the steering wheel is held in the neutral position. This means that the rotary servovalve does not serve to retain a piston of the power cylinder in a neutral position. As a result, the piston of the power cylinder is displaced by a disturbance reaction force applied thereto from a road surface. When applied with such a disturbance reaction force during high speed straight travel of the vehicle, the power cylinder acts to render operation of the steering wheel unstable, and the operator is involved in an insecure condition due to unexpected vibration of the steering wheel.
To overcome the foregoing brawback, an improved rotary servovalve has been proposed in Japanese Patent Early Publication No. 60-56668, which includes, as shown in FIG. 7, a valve rotor 4 having a plurality of circumferentially spaced control lands 7 arranged to close distribution ports 6 connected to opposite fluid chambers of a hydraulic power cylinder P when the servovalve is in a neutral position. In such a rotary servovalve, however, the valve rotor 4 acts to interrupt the flow of fluid discharged from one of the opposite fluid chambers of the power cylinder and supplied into the other fluid chamber when the steering wheel is returned to the neutral position from a steered position. As a result, smooth return operation of the steering wheel to the neutral position may not be effected when the power cylinder is applied with a return force from the road wheels.